Grate and method of burning a granular fuel material

ABSTRACT

The grate is used for burning a granular fuel material, for instance a biomass material, to be fed onto a loading area of the grate while a primary air feed is coming from below the grate. The grate includes a perforated bed floor having a downwardly-sloping upper surface converging towards a discharge opening where char is concentrated as the granular fuel material is burned during operation. The grate also includes an elongated and bottom-perforated char-receiving conduit positioned immediately under the bed floor. The char-receiving conduit has an inlet end positioned under the discharge opening, and an outlet end that is opposite the inlet end. The char-receiving conduit downwardly slopes between the inlet end and the outlet end. A method of burning a granular fuel material is also disclosed. The proposed concept can increase the overall thermal efficiency of a heat generator and reduce gas and particle emissions in the atmosphere.

CROSS-REFERENCE TO PRIOR APPLICATION

The present case claims priority to Canadian Patent Application No.2,771,112 filed on Mar. 21, 2012, the entire contents of which arehereby incorporated by reference.

TECHNICAL FIELD

The technical field relates generally to grates and methods of burninggranular fuel materials so as to produce heat energy with an increasedoverall thermal efficiency as well as a reduction of gas and particleemissions in the atmosphere.

BACKGROUND

Many different models of heat generators have been suggested over theyears for burning fuel, thereby producing heat energy for a givenpurpose. Existing heat generators vary in size, configuration, shape andefficiency, to name just a few of the differences between them. The typeof fuel being used to generate the heat and the heat output requirementsare two examples of factors that generally have an impact on theirdesign.

While maximizing thermal efficiency is almost always one of the goalswhen designing a heat generator, further increasing the thermalefficiency above levels already obtained using existing approaches is acontinuous challenge since this has a direct impact on the operationalcosts. Goals set for reducing gas and particle emissions in theatmosphere also prompts designers to optimize the thermal efficiency,especially in large commercial or industrial installations. One way toexpress the thermal efficiency of a heat generator is to measure theheat energy output per given quantity of material burned therein.

Some heat generators are designed for burning one or more granular fuelmaterials, for instance a biomass material such as corn cobs, sawdust,scrap or loose wood fragments, etc. Many other variants exist. Many suchbiomass materials are often considered waste byproducts and are oftensimply discarded or not used for generating heat. While most such fuelsare not particularly efficient compared to other possible fuels, theyhave the advantage of being generally economical and widely available insome areas, in particular some rural areas.

The heat generators designed for burning granular fuel materials ofteninclude a grate to support the burning fuel and promote air circulationthrough it. A grate generally includes perforations and/or spaced-apartbars. Increasing the available air generally increases the combustionefficiency, i.e. the capacity to burn all fuel matter. However,increasing the air feed can also decrease the overall heat transferefficiency of the heat generator since the temperature of the hot gasesfrom the combustion decreases when the air is in excess. The excess airis also increasing the losses at the chimney by increasing the mass ofunused heated air released in the atmosphere.

Minimizing the excess air is thus highly desirable for maximizing thethermal efficiency. With a greater thermal efficiency, less fuel isneeded and therefore, gas and particle emissions are reduced. Areduction of the excess air can also reduce the amount of particlesbeing carried away out of the chimney.

While the existing approaches for burning granular fuel materials havebeen successful in terms of heat production, there is still room formany improvements in this area of technology, particularly for furtherincreasing the overall thermal efficiency.

SUMMARY

The proposed concept relates to a grate and a method of burning agranular fuel material in which the distribution efficiency of theprimary air is controlled using the grate itself. Unlike existinggrates, the char that forms near the end of the burning process isconcentrated in a conduit located under the perforated bed floor of thegrate, where it burns until only ashes are left. This way, theperforated bed floor can always remain covered with granular fuelmaterials and the primary air is substantially prevented from bypassingthe grate through uncovered perforations of the bed floor.

In one aspect, there is provided a substantially horizontally-disposedgrate for burning a granular fuel material to be fed onto a loading areaof the grate while an air feed is coming from below the grate, the grateincluding: a perforated bed floor having a downwardly-sloping uppersurface converging towards a discharge opening where char isconcentrated as the granular fuel material is burned during operation;and an elongated and bottom-perforated char-receiving conduit positionedimmediately under the bed floor, the char-receiving conduit having aninlet end positioned under the discharge opening, and an outlet end thatis opposite the inlet end, the char receiving conduit downwardly slopingbetween the inlet end and the outlet end.

In another aspect, there is provided a method of burning a granular fuelmaterial, the method including the concurrent steps of loading thegranular fuel material in a loading area of a substantiallyhorizontally-disposed bed floor; vibrating the bed floor to move thegranular fuel material from the loading area towards a discharge openinglocated away from and vertically below the loading area; feeding primaryair across the bed floor, the primary air coming from a bottom side andpassing through a multitude of spaced-apart perforations made in the bedfloor; drying, mostly by radiation heat, the granular fuel materialimmediately after the granular fuel material is loaded onto the bedfloor; transforming, by pyrolysis, the dried granular fuel material intovolatile compounds and char, and generating heat above the bed floor;collecting and concentrating the char passing through the dischargeopening into an elongated chamber extending substantially horizontallyunderneath the bed floor; and generating heat by burning the char insidethe chamber as the char is moved from the discharge opening towards anoutlet end of the chamber by the vibrations of the bed floor.

Further details on these aspects as well as other aspects of theproposed concept will be apparent from the following detaileddescription and the appended figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic view illustrating an example of a generic heatgenerator for burning a granular fuel material;

FIG. 2 is a side view illustrating an example of a grate having aconstruction based on the proposed concept;

FIG. 3 is a top isometric view of the grate of FIG. 2;

FIG. 4 is a view similar to FIG. 3, taken from another angle;

FIG. 5 is an enlarged top view of the grate of FIG. 2;

FIG. 6 is a side view illustrating another example of a grate having aconstruction based on the proposed concept;

FIG. 7 is a top isometric view of the grate of FIG. 6; and

FIG. 8 is a bottom isometric view of the grate of FIG. 6.

DETAILED DESCRIPTION

FIG. 1 is a schematic view illustrating an example of a generic heatgenerator 10 for burning a granular fuel material. This heat generator10 can be used, for instance, as a furnace or a boiler. The heat energycan be transferred to a fluid passing through a heat exchanger or bedirectly used for another purpose, such as to heat a pressure vesselaround which the hot flue gases circulate. Other configurations andarrangements are possible as well.

The heat generator 10 includes a casing 12 inside which a grate 14 isprovided. The grate 14 either fills the entire internal width of thecasing 12, as schematically shown, or is either mounted on a supportingstructure preventing air under the grate 14 from bypassing it around itsperiphery.

The grate 14 is designed to hold the granular fuel material while itburns continuously after being loaded thereon and ignited. Ignition isdone using one or more of the possible ignition methods, as known tothose skilled in the art.

The grate 14 is disposed substantially horizontally, meaning that thegrate 14 is acting as a receptacle over which the granular fuel materialis supported by gravity. An example of granular fuel material is abiomass material, such as corn cobs, sawdust, scrap or loose woodfragments, etc. Coal is another example of a granular fuel material.Many other variants exist. The granular fuel material can be ahomogenous material or a mix of two or more materials, regardlesswhether the expression refers to “material” or “materials”. Also, theterm “granular” as used in the present context means a particle or asmall piece, such as for example but not limited to, having a sizeranging from about a fragment of a few millimeters in length to about acoarse granule of a few centimeters in length, as generally understoodby those skilled in the art. When used with “fuel material” or “fuelmaterials”, the term “granular” refers to a burnable substance that canbe handled in bulk and that is not a gas or a liquid, as generallyunderstood by those skilled in the art.

For the sake of simplicity, the granular fuel material will simply bereferred to as “fuel” from this point onwards.

As schematically shown in FIG. 1, the fuel is fed to the grate 14 from afuel source 16. The grate 14 is configured and shaped to hold a givenquantity of fuel and fuel will cover almost the entire bed floor whenthe heat generator 10 is in operation. The fuel is loaded onto the grate14 at a loading area located on the upper surface of a perforated bedfloor of the grate 14. The fuel falls by gravity onto the bed floor, forinstance coming from an endless screw conveyor. Variants are possible aswell.

Air coming from a primary air source 18 is supplied under the grate 14when the fuel is burning. The primary air source 18 is for instance ablower or any other suitable device. The primary air source 18 generatesa primary air feed 20. The primary air feed 20 reaches the bottom of thegrate 14 and then passes through perforations provided across thethickness of the perforated bed floor because of a pressure differentialbetween both sides thereof. The exact size, shape and spacing of theperforations depend on various factors. The size of the perforationswill depend, among other things, on the size of the fuel pieces. It isof course desirable to prevent fuel pieces from falling by gravitythrough the perforations. Still, the perforations are not necessarilymade or all made with a circular cross section. For instance, theperforations can be oblong or can even have any other shapes, such asrectangular, octagonal, etc. They can also have an irregular shape oreven be tapered. Other variants are possible.

The burning process occurring at the grate 14 generates heat (radiantand convective) as well as gases, among other things. These gases risingfrom the grate 14 still contain inflammable gases in form of volatilecompounds, especially when the primary air feed 20 does not supplyenough oxygen for a complete combustion. An example of volatile compoundis carbon monoxide (CO). The combustion is completed in a zone 22located above the grate 14. This zone 22 receives additional air from asecondary air source 24. The secondary air source 24 generates asecondary air feed 26. The secondary air source 24 is for instance ablower or any other suitable device. Typically, the primary air feed 20is about 35% of the total supplied air and the secondary air feed 26 isthus about 65% of the total supplied air. The relative proportions ofthe primary and the secondary air feed can be controlled manually and/orusing an automated control system. The control system can also modulatethe air flow in function of the amount of fuel being supplied. Otherconfigurations, arrangements and proportions are also possible.

In the illustrated example, the hot gases coming from the zone 22 passthrough a heat exchanger 28 where convective heat energy is collected.The heat exchanger 28 also receives radiant heat from the burning fuel.This heat exchanger 28 has an internal fluid circuit connected to anincoming conduit 30 and an outgoing conduit 32. The outgoing conduit 32sends a heated fluid where or closer to where the heat energy is needed.The incoming conduit 30 and the outgoing conduit 32 can form aclosed-loop circuit and/or an opened-loop circuit, depending on theneeds. Variants are also possible.

In some implementations, one or more additional heat exchangers (notshown) can be provided to recover more heat energy from the gasesdownstream the heat exchanger 28. The gases eventually exit the casing12 as flue gases 34. The flue gases 34 can be discarded through achimney and/or used in another process. It should be noted that the fluegases 34 often contain small particles in suspension. Nevertheless, theywill still be referred to as “gases” for the sake of simplicity.

The illustrated heat generator 10 is designed to be operated in acontinuous manner, meaning that the fuel burns continuously for as longas fuel is supplied or unless the combustion is abruptly stopped forsome reason. Accordingly, fuel is loaded continuously or at givenintervals (regular or not) while the burning process is ongoing. Aportion of the fuel that was put on the perforated bed floor willtransform into granular char and a portion will transform into thevolatile compounds to be burned as well in the zone 22. Typically, about80% of the carbon from the fuel will be transformed in volatilecompounds and about 20% will become char. The combustion of the volatilecompounds forms the visible flames in a fire while the char is seen asglowing red coals or embers which often burn without the presence offlames. On average, the volatile compounds will require about two timesmore oxygen than char to burn.

The char will eventually form ashes and other solid debris that need beremoved from the grate 14. Debris can be, for instance, fragments orpieces (such small rocks, sand, metal fragments, etc.) that cannot burnat the temperatures involved. Other kinds of debris are also possible.For the sake of simplicity, the terms “ash” and “ashes” are meant toinclude debris present therein, if any. In FIG. 1, the ashes are removedfrom the bottom of the grate 14 and out of the casing 12 using an ashremoval system, which system is schematically depicted in FIG. 1 at 36.The ash removal system 36 can include, for instance, an endless screwcarrying the ashes outside for disposal. Other kinds of systems are alsopossible.

In use, the grate 14 is vibrated to progressively move the fuel over theperforated bed floor of the grate 14. The vibrations can be generatedusing a vibrations generator that is connected to or mounted on thegrate 14. The vibrations generator is schematically depicted in FIG. 1at 40. The vibration generator 40 of the illustrated example is locatedoutside the casing 12 and is mechanically connected to the grate 14using a link that is schematically depicted in FIG. 1 at 42. Thisvibration generator 40 could also be located inside the casing 12 insome implementations. Other configurations and arrangements are alsopossible. The grate 14 is supported at its periphery by a suitablesupporting arrangement which, however, is not part of the proposedconcept and does not need to be described furthermore. The vibrationscan be continuous or intermittent, depending on the implementations.

The grate 14 can be made of a material such as a metal or a coated metalcapable of withstanding the temperatures involved over long time periodswhile the fuel is burning. These temperatures can be up to about 800°C., sometimes even more.

If desired, the grate 14 can include an internal cooling circuit, forinstance an internal cooling circuit having a network of conduitsdesigned to keep some of the parts of the grate 14 below a giventemperature. The internal cooling circuit and the associated coolingsystem located outside the casing 10 are schematically depicted in FIG.1 at 50.

The grate 14 can be constructed like the one of the example illustratedin FIG. 2. In FIG. 2, this grate is referred to as the grate 100. Thegrate 100 has a perforated bed floor 102 that is generally conical inshape. Its periphery is also generally circular in shape, as best shownin FIGS. 3 and 4. FIGS. 3 and 4 are both top isometric views of thegrate 100. FIG. 5 is an enlarged top view of the grate 100.

The perforated bed floor 102 of the illustrated grate 100 is made of aplurality of juxtaposed flat panels 104, for example panels weldedtogether along their edges so as to form a downwardly-sloping uppersurface. The average inclination of the bed floor 102 can be generallybetween 5 and 25° with reference to the horizontal, although othervalues are possible as well. The panels 104 form a funnel-like structurethat will hold the fuel when the grate 100 is disposed substantiallyhorizontally. The illustrated grate 100 also includes a circular rim 106located around the periphery of the bed floor 102. The rim 106 has aplurality of axisymmetric holes 108 for connecting the grate 100 to asupporting arrangement or the like.

It should be noted that the bed floor 102 can be constructeddifferently. For instance, one can use a single panel and shape it asdesired in a large press or the like. Other constructions and ways ofmounting the grate 100 inside the casing 10 are also possible.

The grate 100 has a loading area 110. The upper surface of the bed floor102 converges towards a discharge opening 112 that is somewhat locatedaway from the loading area 110. Also, the discharge opening 112 of theillustrated grate 100 is offset with reference to a geometric center ofthe upper surface of the bed floor 102. This was made to maximize thelength of the path of the fuel over the grate 100. Nevertheless, usinganother configuration is also possible.

The discharge opening 112 of the illustrated grate 100 is located withinthe periphery of the upper surface of the bed floor 102, thus inside therim 106. Variants are possible as well. For instance, one can design agrate with a discharge opening 112 that is located at the edge of theperiphery of the bed floor 102.

In use, the various steps of the burning process occur concurrentlysince the fuel burns continuously, unlike for instance a heat generatorusing a liquid fuel or gas fuel for which interrupting the burningprocess is much easier. Once on the bed floor 102, the fuel is vibratedand will progressively move from the loading area 110 towards thedischarge opening 112. The discharge opening 112 is located away fromand vertically below the loading area 110. Thus, using the vibrations,the fuel will progressively move towards that location as it burns.

As best shown in FIG. 2, the grate 100 includes an elongated andbottom-perforated char-receiving conduit 120 positioned immediatelyunder the bed floor 102. The illustrated char-receiving conduit 120 hasan inlet end 120 a which includes an upper opening positioned directlyunder the discharge opening 112, and an open-ended outlet end 120 b thatis opposite the inlet end 120 a. The char-receiving conduit 120downwardly slopes between the inlet end 120 a and the outlet end 120 b.The average inclination can be generally between 5 and 20° withreferenced to the horizontal, although other values are also possible aswell.

The char-receiving conduit 120 is entirely supported by the bed floor102. For instance, the char-receiving conduit 120 of the grate 100 canbe welded or otherwise attached underneath the bed floor 102 around theperiphery of the discharge opening 112. One can also use brackets or thelike, if desired. The bed floor 102, the rim 106 and the char-receivingconduit 120 form a compact monolithic unit. Variants are possible aswell.

In the illustrated example, the char-receiving conduit 120 issubstantially tubular in shape. Nevertheless, other shapes andconfigurations are possible as well. For instance, the char-receivingconduit 120 could have a rectangular cross section or any other shape(oval, triangular, etc.) The size and shape of the char-receivingconduit 120 can also vary along its length. The inlet end 120 a of thechar-receiving conduit 120 includes an inclined end wall panel 122 sothat the char received from the discharge opening 112 can only gotowards the outlet end 120 b. The outlet end 120 b, however, is openended. Alternatively, one can provide a char-receiving conduit 120 withan end wall panel (not shown) at the end 120 b and use a large bottomopening adjacent to the end 120 b as the ash outlet/air intake.

The cross-sectional area of the char-receiving conduit 120 can begenerally about 2 to 4% of the area of the upper surface of the bedfloor 102. These values should provide very good results in mostimplementations. Nevertheless, other values are possible as well.

The illustrated grate 100 has a vertical plane of symmetry depicted byline 130 in FIG. 5. The char-receiving conduit 120 of the grate 100 hasa longitudinal axis that extends substantially parallel to the plane ofsymmetry 130. This way, the heat generated inside the char-receivingconduit 120 will spread evenly on both sides of the bed floor 102.Nevertheless, one can construct a grate that is not symmetrical or notentirely symmetrical.

One of the goals of the grate 100 is to maximize the thermal efficiencyof the heat generator by optimizing the use of the amount of the primaryair from the primary air feed coming from below the grate 100. Theapproach involves using different amounts of air for the differentstages of the burning process occurring on the grate 100. It alsoinvolves the fact that the perforations on the bed floor 102 are alwayscovered with a layer of fuel by way of the concentration of the fuelmatter along the end of the grate 100 and that the concentrated charwill burn under the bed floor 102 in a specially and specificallydesigned component of the grate 100.

The stages of the burning process can be roughly segmented, forinstance, as a drying stage, a pyrolysis stage, a fuel combustion stageand a char combustion stage. While the boundaries between the variousstages are not necessarily clearly visible in practice within the fuelmass, it is possible to predict by mathematical models based on thephysic of combustion of fuel where each stage will approximately happenfor a given type of fuel. The present concept uses this predictabilityto better control the amount of the primary air to be supplied to thefuel and optimizing the solution through the design of the grate 100itself. This is done by selecting one or more perforation patterns ofthe grate 100 instead of using a segmented primary air feed, forexample. A segmented primary air feed generally involves using aplurality of compartments directing different streams of the primary airto specific locations on the underside of a grate. While this approachmay perhaps still be useful in some implementations, it is moredesirable to use only a single primary air feed to lower both costs andcomplexity.

The first stage is the drying stage. Not all fuels necessitate a dryingstage but most biomass fuel materials will require one since they oftenhave relatively high moisture contents. In the drying stage, the fuelmostly uses the intense radiant heat coming from the combustion in thesubsequent stages to evaporate this moisture. Convective heat may alsocontribute to drying the fuel but at a lesser extent. The primary airrequirement is the lowest at the drying stage since there is essentiallyno combustion. The drying stage occurs at and around the loading area110, generally at a temperature of about 100° C.

The next stage is the pyrolysis stage. Pyrolysis can be broadly definedas a thermochemical decomposition of organic material at elevatedtemperatures. Using the oxygen contained in the primary air, the carbonmaterial then transforms itself into char (fixed carbon) and volatilecompounds (volatile carbon). The volatile compounds will generally startforming at about 250° C. The rate of pyrolysis will increase as thetemperature increases in the combustion chamber. The temperature in thecombustion chamber can even reach as high as 1200° C. depending on thetype of fuel used.

The combustion stage occurs after the pyrolysis stage. In the fuelcombustion stage, more air (thus more oxygen) is generally neededcompared to the preceding stages. Generally, the primary air feed iscalculated so that the entire oxygen content of the primary air will beused at the grate 100. The combustion of the volatile compounds willthus be incomplete. The combustion will be completed above the grate 100using the secondary air provided downstream. It should be noted that theproduction of volatile compounds also continues during the combustionstage. It will continue until only char is left. The main differencesbetween the pyrolysis stage and the combustion stage include the amountof oxygen available and the amount of heat being generated.

The grate 100 is designed so that most of the fuel becomes char when itreaches the discharge opening 112. The char then slowly sink into thechar-receiving conduit 120, where it is further concentrated and whereit burns right underneath the bed floor 102. The char-combustion stageis the last stage of the burning process.

Ashes are formed as a result of the combustion of the char and, asaforesaid, exit through the open-ended outlet end 120 b. Ashes generallyrepresent from 1 to 4% of the total mass of fuel provided over the grate100, depending on the fuel grade.

It should be noted that while char is also some fuel by definition, theskilled reader will understand that the distinction between “fuel” and“char” is only made in the context of the transformation of the fuelduring the combustion process.

As shown in FIG. 5, the bed floor 102 of the illustrated grate 100 hasfive different sets of perforation to control the amount of the primaryair passing there through. The perforations of the perforation patternsvary in size, shape, density and/or diversity. Each perforation patternon the bed floor 102 forms what is referred to hereafter as a region.The density refers to the relative spacing between the perforationswhile the diversity refers to the possible combination of two or moredifferent kinds of perforations. Still, one can use identicalperforations in some or even all of the regions. One can also design agrate with fewer regions of distinct perforation patterns. Using asingle region is even possible is some very simple designs.

During operation of the heat generator, fuel is loaded on the perforatedbed floor 102 of the grate 100 up to a given level. Because of thevibrations to which the grate 100 is subjected, the fuel will bescattered over the entire bed floor 102 and the fuel level will tend tobe leveled on the top thereof. The perforations of the bed floor 102 areconstantly covered by some fuel and the thickness of the fuel mass isthus another factor to consider. Constantly covering the perforationswill create an air restriction, especially if the fuel pieces arerelatively small as they will be more densely packed than larger ones,thereby preventing the primary air from by-passing the grate 100 tocreate excess air diluting the hot gases above the grate 100. Thisapproach will greatly improve the overall thermal efficiency.

In the illustrated grate 100, the first region is adjacent to theperiphery of the upper surface of the bed floor 102 and includes theloading area 110. This first region corresponds approximately to thedrying stage for the fuel pieces that are near the upper surface of thebed floor 102.

It should be noted that during operation, fuel is loaded, as aforesaid,up to a given level on the perforated bed floor 102. Therefore, freshfuel arriving at the loading area 110 over the fuel mass already presentwill not necessarily follow a straight line from the loading area 110 tothe discharge opening 112. For instance, some fuel pieces will ratherfollow an arcuate path near the top of the fuel mass. The flow of fuelon the bed floor 102 is tridimensional in nature when considering thefuel mass. The design of the grate 100 takes into account the time takenby all fuel pieces to travel from the loading area 110 down to thedischarge opening 112. This is the reason why the first region (which istwo-dimensional in nature) relates to the corresponding perforationpattern and is only indicative of where the drying stage approximatelyoccurs for the fuel pieces that are near the upper surface of the bedfloor 102.

The second region of the illustrated grate 100 is located closer to thedischarge opening 112 and surrounds the periphery of the first region.It corresponds approximately to the pyrolysis stage for the fuel piecesthat are near the upper surface of the bed floor 102.

As can be seen in FIG. 5, the first region and the second region arewider along the plane of symmetry 130 than on their sides. This takesinto account the fact that fuel pieces will tend to travel more quicklywhen they are near the upper surface of the bed floor 102 compared tofuel pieces at the top of the fuel mass. Another factor that can betaken into account is the heat. Fuel pieces receiving more heat thanothers will dry faster and complete the pyrolysis stage faster, forinstance.

The third region of the illustrated grate 100 is located between theperiphery of the second region and the periphery of the dischargeopening 112. This third region corresponds approximately to a portion ofthe fuel combustion stage for the fuel that is near the upper surface.The fourth and fifth regions are located further away from the loadingarea 110. They correspond approximately to other portions of thecombustion stage for the fuel pieces that are near the upper surface ofthe bed floor 102.

As aforesaid, the transformation of the fuel into the volatile compoundsand the char is completed about the time the fuel (now in the form ofchar) reaches the discharge opening 112. Char pieces will then fill theentire width of the discharge opening 112 and will slowly progress intothe char-receiving conduit 120.

The char-receiving conduit 120 has bottom perforations to receiveprimary air. These perforations can include one or more differentpatterns. For instance, the perforations near the outlet end 120 b canbe smaller to prevent the progressively smaller char pieces and theashes from falling through. Air can also enter the char-receivingconduit 120 through the outlet end 120 b. Thus, as a skilled reader willunderstand in the context, the outlet end 120 b of the char-receivingconduit 120 is also a primary air inlet and the discharge opening 112 isalso a primary air and combustion gases outlet.

In use, the char will progress along the char-receiving conduit 120because of the vibrations to which the grate 100 is subjected. The gasesresulting from the combustion of the char will escape through thedischarge opening 112. The thickness of the char layer inside theconduit 120 will diminish progressively from the inlet end 120 a to theoutlet end 120 b. The perforations of the char-receiving conduit 120near the outlet end 120 b will only be covered by a progressivelythinner layer of char. Some perforations may also be completelyuncovered. Nevertheless, the presence of concentrated char inside theinlet end 120 a of the conduit 120 (thus, inside the discharge opening112) will prevent the primary air from flowing in large quantitiesacross the discharge opening 112. Thus, unlike existing grates, thelocation where the char burns will not generate unused primary air thatwould only increase the excess air and lower the overall thermalefficiency.

Still, providing the char under the bed floor 102 in a concentratedmanner will maximize the heat. The char will also consume a good amountof the oxygen from the primary air. The heat generated therein will betransferred to the bed floor 102 through radiant heat and also someconvective heat. The primary air coming through the perforations of thebed floor 102 and from the discharge opening 112 will already bepre-heated to some extent.

In the example shown in FIGS. 2 to 5, the grate 100 is designed so thatthe overall primary air feed passageway of the first region will beabout 6% of the primary air feed. The second, third, fourth and fifthregions will provide about 19%, 25%, 30% and 16% of the total primaryair feed, respectively. The balance (4%) will come through the dischargeopening 112. Other designs, configurations and proportions are possibleas well.

FIG. 6 is a side view illustrating another example of a grateincorporating the proposed concept and for use in a heat generator suchas the heat generator 10 shown in FIG. 1. This grate referred to as thegrate 200. It includes a perforated bed floor 202 having an uppersurface with a rim 204 around its periphery. The perforated bed floor202 is generally rectangular in shape, as best shown in FIGS. 7 and 8.FIGS. 7 and 8 are a top isometric view and a bottom isometric view ofthe grate 200, respectively. The perforated bed floor 202 is made usinga plurality of flat panels 206 that are welded together. Variants arealso possible. The grate 200 has a loading area 208.

The grate 200 includes two discharge openings 210 and two char-receivingconduits 212 with bottom perforations. This feature can also beimplemented on another kind of grate, such as the grate 100. Thedischarge openings 210 of the grate 200 are disposed side-by-side andare located adjacent to the rim 204 of the perforated bed floor 202.

Still, one can design the grate 200 with only one discharge opening 210and only one char-receiving conduit 212. Other possible configurationsand arrangements include using more than two discharge openings 210 andmore than two char-receiving conduits 212, and/or using a grate havingtwo or more spaced-apart loading areas converging towards one or moredischarge openings 210. Still, one can use two or more char-receivingconduits 212 with only one discharge opening 210. Many othercombinations are possible as well.

Also shown in FIGS. 6 and 8 are examples of brackets 220 for attachingthe char-receiving conduits 212 underneath the bed floor 202 of thegrate 200. Variants are also possible.

As can be appreciated, the proposed concept provides a way to bettercontrol the amount of primary air when generating heat energy using agranular fuel material fed on a grate. It also provides a way ofdesigning a grate that is very compact. This grate can improve theoverall thermal efficiency of a heat generator.

The present detailed description and the appended figures are meant tobe exemplary only. A skilled person will recognize that variants can bemade in light of a review of the present disclosure without departingfrom the proposed concept.

What is claimed is:
 1. A substantially horizontally-disposed grate forburning a granular fuel material to be fed onto a loading area of thegrate while an air feed is coming from below the grate, the grateincluding: a perforated bed floor having a downwardly-sloping uppersurface converging towards a discharge opening where char isconcentrated as the granular fuel material is burned during operation;and an elongated and bottom-perforated char-receiving conduit positionedimmediately under the bed floor, the char-receiving conduit having aninlet end positioned under the discharge opening, and an outlet end thatis opposite the inlet end, the char-receiving conduit downwardly slopingbetween the inlet end and the outlet end.
 2. The grate as defined inclaim 1, wherein the bed floor has at least two regions with distinctperforation patterns, a first region among the at least two regionsbeing adjacent to the periphery of the upper surface and including theloading area, and a second region among the at least two regions beinggenerally located between the first region and the discharge opening 3.The grate as defined in claim 2, wherein the perforation pattern of oneof the at least two regions creates a larger overall primary air feedpassageway than the perforation pattern of another of the at least tworegions.
 4. The grate as defined in claim 1, wherein the dischargeopening is located within the periphery of the upper surface.
 5. Thegrate as defined in claim 4, wherein the discharge opening is offsetwith reference to a geometric center of the upper surface.
 6. The grateas defined in claim 1, wherein the upper surface of the bed floor isgenerally conical in shape.
 7. The grate as defined in claim 1, whereinthe periphery of the upper surface is generally circular in shape. 8.The grate as defined in claim 1, wherein the periphery of the uppersurface is generally rectangular in shape.
 9. The grate as defined inclaim 1, wherein the char-receiving conduit is substantially tubular inshape.
 10. The grate as defined in claim 1, wherein the char-receivingconduit is substantially rectangular in shape.
 11. The grate as definedin claim 1, wherein the bed floor has a vertical plane of symmetry andthe char-receiving conduit generally has a longitudinal axis thatextends substantially parallel to the plane of symmetry.
 12. The grateas defined in claim 1, wherein the char-receiving conduit is supportedunder the bed floor using at least one bracket.
 13. The grate as definedin claim 1, further including at least one additional discharge openingand at least one additional char-receiving conduit, one for eachadditional discharge opening, the discharge openings being spaced apartfrom one another and the char-receiving conduits extending substantiallyparallel to one another.
 14. The grate as defined in claim 1, whereinthe conduit has a cross-sectional area that is about 2 to 4% of an areaof the upper surface of the bed floor.
 15. The grate as defined in claim1, wherein the upper surface has an average slope between 5 and 25degrees with reference to the horizontal.
 16. The grate as defined inclaim 1, wherein the char-receiving conduit has an average slope between5 and 20 degrees with reference to the horizontal.
 17. A method ofburning a granular fuel material, the method including the concurrentsteps of: loading the granular fuel material in a loading area of asubstantially horizontally-disposed bed floor; vibrating the bed floorto move the granular fuel material from the loading area towards adischarge opening located away from and vertically below the loadingarea; feeding primary air across the bed floor, the primary air comingfrom a bottom side and passing through a multitude of spaced-apartperforations made in the bed floor; drying, mostly by radiation heat,the granular fuel material immediately after the granular fuel materialis loaded onto the bed floor; transforming, by pyrolysis, the driedgranular fuel material into volatile compounds and char, and generatingheat above the bed floor; collecting and concentrating the char passingthrough the discharge opening into an elongated chamber extendingsubstantially horizontally underneath the bed floor; and generating heatby burning the char inside the chamber as the char is moved from thedischarge opening towards an outlet end of the chamber by the vibrationsof the bed floor.
 18. The method as defined in claim 17, wherein duringcontinuous operation, the perforations of the bed floor are constantlycovered by some of the granular fuel material.
 19. The method as definedin claim 17, wherein feeding the primary air across the bed floorincludes passing the primary air through at least two different sets ofperforations in the bed floor, a first set among the at least two setsproviding less air across the bed floor than a second among the at leasttwo sets.
 20. The method as defined in claim 17, wherein feeding theprimary air across the bed floor includes passing some of the primaryair through the discharge opening while the discharge opening is filledwith some of the char.
 21. The method as defined in claim 17, furtherincluding: feeding secondary air above the bed floor to complete thecombustion of the volatile compounds.